1. Field of the invention
The present invention relates to a method for propelling droplets of an electrically conductive liquid according to which the end of a first electrode whose cross-section is approximately of the order of size of that of the droplets is disposed in this liquid, this end being flush with an insulated support surrounded by the said liquid a second electrode, a surface of which is substantially greater than that of the said end of the first electrode, is disposed in this liquid in contact with it, and these two electrodes are connected to the terminals of a pulse generator to cause resistive heating of the liquid in the immediate proximity of the said end, suitable for vaporising a quantity of the said liquid capable of producing a force able to propel a droplet of this liquid.
2. Description of the prior art
A structure capable of effecting such a method is described in European Patent Specification No. B1 0,106,802. Study of the manner of energising such a structure has s the results and the efficiency vary appreciably depending on the mode of energisation chosen. Thus, in French Patent Specification No. 2,092,577 it has been proposed to connect two electrodes submerged in liquid ink to a high voltage source to form a discharge circuit in such a manner as to create a spark which generates an over-pressure within the liquid, causing it to be ejected through an opening. Such a mode of energisation has disadvantages linked to the use of a high voltage source, the principal disadvantage arising however from the poor efficiency resulting from this mode of propulsion of liquid droplets.
The use of much lower voltages has shown that it is also possible to propel droplets of liquid by generating within the mass of liquid a force resulting from the vaporising of a volume of liquid in the neighbourhood of the end of an electrode aligned with the surface of an insulating support surrounded by the liquid droplets of which are to be propelled. Detailed study of the phenomenon has shown, on the basis of measurements that there exists a range of voltages for which an appropriate volume of liquid is vaporised. However, the vaporisation alone of this liquid in accordance with Ohms law is not sufficient to produce the propulsion energy necessary for the droplet. It has been remarked, however, that if the voltage is sufficient, as soon as the current tends to break-off, it is quickly re-established as a result of what may be interpreted as a sort of ionisation of the liquid vapour.
While this mode of propulsion shows itself to be effective and relatively efficient compared to other modes of propulsion of droplets on demand, used in particular in ink jet printing systems poor reproducibility of that phase of the process of propulsion which may be termed "ionisation" has also been noticed which shows as a great variation in the size of the droplets, from being equal to at least double, between the projection of two successive droplets. It is very evident that such a variation is not desirable, in particular when these droplets are intended to form characters in an ink jet printing system.
It has already been proposed in U.S. Pat. Specification No. 4,746,937 to limit the energy in a very different ink jet system, in which the conductive ink is disposed in a long tube and fulfills the role of a heating resistance. In this ink jet, a volume of ink corresponding to several tens of times the volume of ink to be expelled is heated in such a way that if the heating conditions are kept constant, a stage is arrived at where the total volume of the tube is emptied as a result of constant increase in the temperature of the ink contained in this tube. It is for this reason that it has been proposed to control the duration of the ink preheating pulse in such a manner that it is inversely proportional to the initial temperature of the ink. This solution is of no great interest when the volume of ink heated is more or less equal to that expelled, such that the following volume of ink is more or less at ambient temperature. Thus this solution does not tackle the problem which concerns us.
It has also been proposed in U.S. Pat. Specification No. 4,126,867 to limit the polarising voltage of the base of an amplifying transistor whose emitter is connected to a piezo-electric motor element, but this does not advantageously tackle the problem which concerns us.